An exhaust turbocharger comprises a turbine driven by the exhaust gas stream, and a compressor driven by the turbine to increase the volume of intake air in to the engine. As a result more fuel can be burnt in each combustion event, and the power output can be increased. Such turbochargers are a common feature on internal combustion engines.
A simple exhaust turbocharger is characterised by a fixed aspect ratio for the turbine. This aspect ratio can be optimum for only one engine speed and load, and accordingly requires a compromise between lack of boost at low engine speed/low exhaust gas flow rate, and too much boost at high engine speed/high exhaust gas flow. In the latter case, excess boost can be relieved via a wastegate, but energy is not then recovered from the wasted stream.
Variable geometry exhaust gas turbochargers have been proposed whereby the turbine volute geometry is altered in accordance with engine speed. This arrangement allows the geometry to be optimised to some extent across a range of engine speed, and may permit the turbo charger to be effective at both low and high engine speeds.
Variable geometry turbochargers may provide better space utilisation than two or three stage turbochargers, where gas is for example directed sequentially to one turbine after another according to engine speed, each turbocharger being optimised for a particular range of engine speed.
Typically a variable geometry turbocharger incorporates a nozzle arrangement for directing the exhaust gas stream onto the turbine, and the angle of incidence of the exhaust gas stream is changed according to engine speed. Actuation of the nozzle arrangement may be for example by vacuum actuator or electric stepper motor.
At lower engine speeds, the angle of incidence is more orthogonal to the blade of the turbine, and at higher engine speeds, the angle of incidence is less orthogonal. However, at lower engine speeds, and an optimum angle of incidence, the back pressure generated upstream of the turbine may be significant enough to prevent effective expulsion of combustion gases from the engine on the exhaust stroke. In turn this may reduce the knock margin in a gasoline engine. The alternative is to lower the back pressure upstream of the turbine, but this has the effect of reducing energy recovery from the exhaust gas stream.
A further problem in gasoline engines with exhaust turbochargers is the requirement to separate the exhaust pulsations in a multi-cylinder reciprocating piston engine. This is necessary to avoid interaction of pressure waves as respective exhaust valves open, which may detrimentally affect cylinder scavenging on the exhaust stroke. One solution to this problem is to use a pulse divided manifold which separates the exhaust tracts of cylinders that may interfere (according to the firing order), and directs the two exhaust tracts to different inlet channels of the turbocharger turbine—a so-called twin-scroll turbocharger.
In an internal combustion engine, the exhaust event may be considered as comprising two sequential phases. Firstly a high pressure pulse occurs as the exhaust valve opens, and combustion chamber pressure drops rapidly—this phase may be termed “blow-down”, and has a short time span.
Subsequently the exhaust gases are expelled from the cylinder/combustion chamber on the exhaust stroke at a lower pressure—this phase may be called “expulsion” and has a relatively long time span. A conventional variable geometry exhaust turbine tends to adversely affect expulsion.
GB-A-2423797 (Lotus) discloses a multi-cylinder internal combustion engine having two exhaust valves per cylinder, the exhaust valves having respective exhaust tracts connected one each to an exhaust turbocharger and to an exhaust turbo compounder.